All right, Grados are low impedance and require little gain. Right off the bat, forget OTL: they are happy only with high impedance headphones.
With tubes, the possibilities for Grados are either hybrid or transformer coupled. If you feel confident, you can try the Starving Student: it's not costly, and I heard it sounds good, but the PCB is no longer available. I've seen a lot of love for the Cavalli Audio EHHA, which will prboably be my next project, and the new PCBs are fresh out of the oven. Other alternatives are the SOHA 2, and the Millett Max and MiniMax (the latter comes with a custom case, which is useful for a beginner, but isn't the beast amp I've heard).
If you want to go full tube, I know of two transformer-coupled amps: the Bottlehead S.E.X and the Hagerman Castanet (my favorite of the amps I built).They cost more, because of the transformers and chokes, but I simply love the sound. An advantage is that both are easy to case, and one comes in a full-kit. Another advantage is that there's none of that "transistor" nonsense
.
For solid-state, I'm affraid I know little. There's the B22 that's supposed to be a reference-grade amp, but that's about all I know.